Checking the generator voltage (BMW 5 Series E39)

Connect a voltmeter between the positive and negative terminals of the battery.

Start the engine. When starting the engine, the battery voltage may drop to 8 volts (at an ambient temperature of +20°C).

Increase the engine crankshaft speed to 3000 min⁻¹. The voltage should be between 13.5 and 14.5 volts. This is proof that the generator and voltage regulator are working properly. To ensure recharging during operation, the generator voltage (on-board voltage) must be higher than the battery voltage.

Check the stability of the voltage regulation. To do this, turn on the high beam and repeat the measurement at an engine crankshaft speed of 3000 1/min. The measured voltage value should not exceed the value obtained before by more than 0.4 volts.

If the measured voltage values are outside the specified values, the generator must be checked in a specialized workshop.

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